1. Field of the Invention
The present invention relates generally to a method and apparatus for retransmission on an UpLink (UL) in a wireless communication system, and more particularly, to a method and apparatus for controlling retransmission on a UL in a wireless communication system supporting multi-antenna transmission technology, such as Multiple Input Multiple Output (MIMO).
2. Description of the Related Art
Wireless communication systems have evolved into broadband wireless communication systems providing not only voice-oriented services but also high-speed, high-quality packet data services, including communication standards such as, for example, 3GPP High Speed Packet Access (HSPA), Long Term Evolution (LTE), 3GPP2 High Rate Packet Data (HRPD), Ultra Mobile Broadband (UMB), and IEEE 802.16e.
Recently, in order to improve transmission efficiency, wireless communication systems use technologies such as Adaptive Modulation and Coding (AMC) and channel-sensitive scheduling. By using AMC, a Node B (also known as a base station) may adjust the amount of data transmitted by the Node B or a User Equipment (UE), also known as a mobile station, according to channel states. For example, if the channel state is poor, the amount of transmission data is reduced to a desired level to match a reception error rate, and if the channel state is good, the amount of transmission data is increased to effectively transmit as much information as possible, while matching the reception error rate to the desired level. By using a channel-sensitive scheduling resource management method, a Node B may selectively service users with a good channel state among a plurality of users, which contributes to an increase in system capacity, as compared to an existing method of allocating a channel to a single user and servicing the user. Specifically, the AMC and the channel-sensitive scheduling are methods of applying an appropriate Modulation and Coding Scheme (MCS) at the time determined to be most efficient, using the channel state information.
Many studies are being conducted to replace Code Division Multiple Access (CDMA), a multiple access scheme which has been used in the 2nd Generation (2G) and 3rd Generation (3G) mobile communication systems, with Orthogonal Frequency Division Multiple Access (OFDMA) in the next-generation communication system. Standards bodies such as 3GPP, 3GPP2, and IEEE are standardizing evolved systems using OFDMA or modified OFDMA. It is well known that a greater capacity increase can be expected in OFDMA, compared to in CDMA. One of several reasons leading to this capacity increase in OFDMA is the possibility of performing scheduling on the frequency domain (known as ‘Frequency-Domain Scheduling’). Just as the capacity gain can be obtained by the channel-sensitive scheduling using the time-varying characteristics of channels, a higher capacity gain may be obtained using the frequency-varying characteristics of channels.
An LTE system, a typical example of the broadband wireless communication systems, adopts Orthogonal Frequency Division Multiplexing (OFDM) in a Downlink (DL) and Single Carrier Frequency Division Multiple Access (SC-FDMA) in a UL, both of which may perform the frequency-domain scheduling.
The AMC and channel-sensitive scheduling are techniques capable of improving the transmission efficiency when transmitters have acquired sufficient information about transmission channels. In the DL of the LTE system, for Frequency Division Duplex (FDD), since a Node B cannot estimate a state of a DL channel or a transmission channel depending on a UL channel or a reception channel, a UE is designed to report information about the DL channel to the Node B. In the case of Time Division Duplex (TDD), a Node B uses characteristics so that it can estimate a state of a DL channel depending on a UL channel, making it possible to omit the process of reporting the information about the DL channel from the UE to the Node B.
In the UL of the LTE system, a UE is designed to transmit a Sounding Reference Signal (SRS) and a Node B is designed to estimate a UL channel by receiving the SRS.
In the DL of the LTE system, MIMO or a multi-antenna transmission technique is supported. An LTE Node B may include 1, 2, or 4 transmission antennas. When including a plurality of transmission antennas, a Node B may obtain a beamforming gain and a spatial multiplexing gain by applying precoding.
Recently, many discussions have been held in 3GPP to support MIMO even in the UL of the LTE system. Similar to the DL MIMO, a UE may include 1, 2, or 4 transmission antennas, and when including a plurality of transmission antennas, a UE may obtain a beamforming gain and a spatial multiplexing gain by applying precoding.
A difference between the DL MIMO and the UL MIMO is provided below. In the DL MIMO, a Node B (or a transmitter) determines by itself the transmission property such as MCS scheme, MIMO scheme, and precoding. The Node B configures and transmits a Physical Downlink Shared CHannel (PDSCH) by reflecting the transmission property, and delivers the transmission property applied to the PDSCH to a UE using a Physical Downlink Control CHannel (PDCCH). However, in the UL MIMO, a Node B (or a receiver) determines the transmission property such as MCS scheme, MIMO scheme, and precoding, according to the channel characteristics of UEs. The Node B delivers the transmission property to a UE through a PDCCH, and upon receiving the PDCCH, the UE configures and transmits a Physical Uplink Shared CHannel (PUSCH) by reflecting the transmission property granted by the Node B. Specifically, in the LTE system, a Node B determines AMC, channel-sensitive scheduling, MIMO precoding, etc., and a UE receives a PDSCH transmitted based on the determination, or configures and transmits a PUSCH according to the determination.
If a Node B has correct information about a channel state, the Node B may determine an amount of transmission data, which is most appropriate for the channel state, using AMC. In actual communication environments, however, there is a significant difference between the channel state that the Node B is aware of, and the actual channel state, due to the estimation error, the feedback error, and the like. Therefore, despite the use of AMC, the transmitter and the receiver may not actually prevent errors from occurring. The majority of wireless communication systems, including the LTE system, employ Hybrid Automatic ReQuest (HARQ), in which, if a decoding failure occurs in an initial transmission, a physical layer immediately retransmits the failed data. HARQ refers to a scheme, in which, if a receiver has failed to correctly decode data, the receiver transmits NACK information indicating the decoding failure to a transmitter, allowing the transmitter to retransmit the failed data in a physical layer. On the contrary, if the receiver has correctly decoded data, the receiver transmits ACK information indicating the decoding success to the transmitter, allowing the transmitter to transmit new data.
In a wireless communication system using the HARQ, because a receiver may improve its reception performance by combining a retransmitted signal with a previously received signal, the receiver stores in its memory the data which was received previously but failed to be decoded, just in case of retransmission.
In order to enable a transmitter to transmit other data for the time required when a response signal from a receiver, such as ACK and NACK, is delivered up to the transmitter, an HARQ process is defined. In accordance with the HARQ process, the receiver may determine whether to combine a previously received signal with a newly received signal using a HARQ Process Identification (HARQ PID). HARQ is classified into synchronous HARQ and asynchronous HARQ according to whether a transmitter provides the HARQ PID to a receiver as a control signal in the HARQ process. In the synchronous HARQ, a transmitter uses a serial number of a subframe carrying a PDCCH, instead of providing a HARQ PID to a receiver as a control signal. The subframe refers to a resource allocation unit in the time domain. However, in the asynchronous HARQ, a transmitter provides a HARQ PID to a receiver as a control signal. The LTE system employs asynchronous HARQ in the DL and synchronous HARQ in the UL.
FIG. 1 illustrates a synchronous HARQ operation in a UL.
Referring to FIG. 1, if a Node B grants a resource allocation for a UL transmission using a PDCCH in an n-th subframe of a DL in step 101, a HARQ PID is determined as resource allocation information by a subframe serial number ‘n’. For example, if a HARQ PID corresponding to a subframe serial number ‘n’ is assumed to be ‘0’, a HARQ PID corresponding to a subframe serial number ‘n+1’ may be defined as ‘1’. A PDCCH for a UL grant, transmitted in a subframe with a serial number ‘n’, includes a New Data Indicator (NDI). If an NDI has been toggled from its previous NDI value, the relevant UL grant is set to allocate a PUSCH for new data transmission. If an NDI has maintained its previous NDI value, the relevant UL grant is set to allocate a PUSCH for retransmission of the previously transmitted data.
If an NDI associated a UL grant is assumed to be toggled in step 101, a UE performs initial transmission on a PUSCH for new data transmission in a subframe #(n+4) in step 103. Whether the Node B has successfully decoded the PUSCH data transmitted by the UE in the subframe #(n+4) is determined using a Physical HARQ Indicator CHannel (PHICH) that the Node B transmits in a subframe #(n+8) in step 105. If it is determined that the PHICH has transmitted a NACK, the UE performs retransmission on the PUSCH in a subframe #(n+12) in step 107. In this way, in the synchronous HARQ, initial transmission and retransmission of the same Transport Block (TB) are performed in sync with serial numbers of subframes.
As described in FIG. 1, the Node B and the UE may normally perform a HARQ operation without introducing a separate HARQ PID, because an agreement was made in advance that the TB having undergone initial transmission in the subframe #(n+4) is retransmitted in the subframe #(n+12). In the example of FIG. 1, since a transmission interval of the same TB includes 8 subframes, the maximum number of HARQ processes capable of running at the same time may be limited to 8.
In the UL synchronous HARQ operation described in FIG. 1, retransmission may be granted using a PHICH capable of indicating only the ACK/NACK signal. If the Node B desires to change the transmission property of a PUSCH, such as a transmission resource and an MCS scheme, in retransmission, the Node B may grant transmission of a PDCCH indicating the change. This HARQ scheme, granting a change in the transmission property of the PUSCH, is called ‘adaptive synchronous HARQ’.
FIG. 2 illustrates an adaptive synchronous HARQ operation in a UL.
Referring to FIG. 2, steps 101 to 105 in FIG. 2 are identical in operation to their corresponding steps in FIG. 1.
In step 105 in FIG. 2, a Node B informs a UE that it has failed to successfully decode the PUSCH transmitted in the subframe #(n+4) in step 103, by delivering a NACK using a PHICH in a subframe #(n+8). In order to change the transmission property during PUSCH retransmission, the Node B transmits a PDCCH including information for changing the transmission property of a PUSCH, together with the PHICH in step 106. The UE may receive the PDCCH including information for changing the transmission property of a PUSCH, because it attempts to receive and decode a PDCCH in every subframe. In step 108, the UE performs retransmission on a PUSCH in a subframe #(n+12) by applying the transmission property indicated by the PDCCH.
According to the above-described adaptive synchronous HARQ, the information for changing the transmission property of a PUSCH is transmitted over a PDCCH. Therefore, if a change in the transmission property of a PUSCH is required during retransmission, a Node B transmits a PDCCH together with a PHICH despite an increase in the amount of DL control information. When maintaining the previous transmission property of a PUSCH, the Node B transmits only the PHICH.
FIG. 3 illustrates an adaptive synchronous HARQ operation of a Node B in a UL.
Referring to FIG. 3, in step 131, a Node B performs UL scheduling to determine a UE to be granted transmission of a PUSCH, and a resource to be used for the PUSCH transmission. In step 133, the Node B transmits a PDCCH to inform the scheduled UE of grant information of the PUSCH. In step 135, the Node B demodulates and decodes the PUSCH, which has been received four subframes after a time when the PDCCH was transmitted in step 133. In step 137, the Node B determines whether the decoding of the PUSCH is successful. If successful, the Node B transmits an ACK to the UE in step 139, and then returns to step 131 to perform new scheduling. On the other hand, if the decoding is failed in step 137, the Node B transmits a NACK to the UE in step 141.
Thereafter, in accordance with an adaptive synchronous HARQ operation, the Node B determines in step 143 whether it desires to change the transmission property of the PUSCH to be different from that designated in step 133. If it is desired to change the transmission property, the Node B transmits a PDCCH including information indicating a new transmission property to be applied for retransmission of the PUSCH in step 145. After indicating retransmission of the PUSCH in steps 143 and 145, the Node B returns to step 135 to receive and decode the retransmitted PUSCH.
FIG. 4 illustrates an adaptive synchronous HARQ operation of a UE in a UL.
Referring to FIG. 4, a UE attempts to receive and decode a PDCCH for a UL grant in step 151, and determines in step 153 whether the decoding of the PDCCH is successful. If successful, the UE determines in step 155 whether an NDI indicating transmission/non-transmission of new data has been toggled. If the NDI has been toggled, meaning that the relevant grant indicates initial transmission of a new TB, then the UE transmits a PUSCH carrying a new TB by applying the transmission property indicated by the PDCCH in step 157. However, if the NDI has not been toggled in step 155, meaning that the relevant grant indicates retransmission with the transmission property changed because a Node B has failed to successfully decode the previous TB having the same HARQ PID, then the UE retransmits a PUSCH carrying the previous TB by applying the transmission property indicated by the PDCCH in step 159. If the UE has failed to successfully decode the PDCCH for a UL grant in step 153, the UE attempts to receive and decode a PHICH in step 161. In step 163, the UE determines if an ACK has been received over the PHICH. Upon receiving the ACK, the UE stops the transmission of the PUSCH in step 165. However, upon receiving a NACK from the PHICH, the UE retransmits a PUSCH carrying the previous TB by applying the transmission property indicated by the last received PDCCH in step 167.
Although the synchronous HARQ has been proposed to enable retransmission by a UE by transmitting only the PHICH without transmitting the PDCCH, when the PDCCH should be transmitted together with the PHICH to indicate the transmission property such as a precoding scheme of a UE, the above resource saving effects may not be expected in the synchronous HARQ. Specifically, while the PHICH carries only the ACK/NACK information, the PDCCH includes various control information for UL transmission in a UE. Therefore, to transmit the PDCCH, a Node B should consume more frequency resources and transmission power. If the PDCCH is to be transmitted to indicate the transmission property, such as a precoding scheme for MIMO transmission, during retransmission in a UL, the consumption of resources for control information increases, requiring a method for reducing a transmission load of control information for retransmission in a UL.